A modern phone shoots 12 to 48 megapixels: 4000+ pixels wide, 5–15 MB per photo. For viewing on a screen, that's massive overkill — a 1600-pixel-wide JPG at 80% quality looks identical in an email, a chat or a document, and weighs a few hundred kilobytes.
The presets above apply exactly that. Drop a whole batch at once, then download everything as a ZIP. Need finer control — say 2000 px for a photo the recipient might print? Just change the max-width field; the files reconvert instantly. Everything happens on your device: your photos are never uploaded to any server.